The present invention relates to a method and apparatus for testing semiconductor lasers, and more specifically relates to a method and apparatus for wafer-level testing of vertical cavity surface emitting lasers prior to their complete assembly.
Semiconductor lasers in use today can generally be classified as edge-emitting diode lasers and vertical cavity surface emitting lasers (xe2x80x9cVCSELsxe2x80x9d). In an edge-emitting laser, a semiconductor gain medium, for example a quantum-well semiconductor structure, is formed as a region deposited on a semiconductor substrate, or wafer. Many devices are typically formed from one single wafer, and once an individual device has been detached from the wafer, mirrors are formed or otherwise positioned on opposite edges of the gain medium, perpendicular to the substrate surfaces. The assembly forms a resonant cavity within which the gain medium is located. Electrical or optical pumping of the gain medium generates a laser beam which propagates in a direction along the plane of the substrate. Edge-emitting lasers thus generate a beam in a direction along the plane of a substrate forming the laser, exiting the device at an edge where the devices are separated into individual units. It is thus not practical to test these devices prior to separating them into individual units, thereby exposing the edges from which the beams are output.
VCSEL""s, in contrast, generate output beams in a direction perpendicular to the plane of a substrate on which they are formed. Thus the orientation of individual VCSELs on a wafer substrate, prior to being separated from one another, is potentially suitable for testing before carrying out the manufacturing steps that lead to separation. Conventional testing methods used on VCSELs involve electrically probing the optical aperture side of a wafer, and detecting light emitted from that side while shorting the opposite side of the wafer to ground. Conventional testing methods and devices do not provide a way to screen and map optical characteristics of epitaxial wafers prior to processing the wafer beyond epitaxial growth.
The present invention is directed to an apparatus for testing properties of a wafer having disposed thereon layers that form all or part of a lasing cavity, the apparatus comprising a chuck receiving a wafer to be tested and a pump light source directing an output beam toward selected locations on a wafer received on the chuck in combination with a laser light detector detecting light emitted from the wafer and a pump light source aiming mechanism selectively varying a position at which the pump light output beam enters the wafer.
In another aspect, the invention is directed to a method of wafer-level testing of semiconductor laser devices, comprising the steps of positioning a wafer to be tested in a predetermined position relative to a pump light source, optically pumping preselected regions of the wafer with the pump light source, and analyzing light emitted from each of the preselected regions to determine the laser-related characteristics of the preselected regions of the wafer.